Library: fix overuse of FormData in XmlHttpRequest (fixes #234)
Library: fix overuse of FormData in XmlHttpRequest (fixes #234)

Use a FormData object only when files are transferred.
Otherwise, send the data using the simpler
'application/x-www-form-urlencoded' content type.

alainfrisch/js_of_ocaml,alainfrisch/js_of_ocaml,alainfrisch/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,alainfrisch/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,alainfrisch/js_of_ocaml,OCamlPro-Henry/js_of_ocaml